1. Field of the Invention
The present invention relates generally to a thermal bimorph. More particularly, the present invention relates to a micromachined thermal bimorph that displaces laterally.
2. Background of the Invention
Current battery-powered embedded sensor systems often require a low power method of determining when a certain level of temperature has been reached. Typical applications, such as in transportation and shipping monitoring, heating and air conditioning, and food storage, would benefit from the ability to monitor the temperature environment with a completely unpowered sensor. In addition, these applications would benefit from the ability to poll that sensor to determine if a temperature extreme was reached, and then reset the sensor for later use. In either case, an ultra-low power sensor, or even a sensor that consumes no quiescent power, would reduce the overall system power consumption enough to allow embedded sensors to operate for decades in portable battery powered applications, or in systems that scavenge small amounts of power from the environment.
A micromachined thermal bimorph can perform the function of moving a set of miniature contacts into intimate contact when a certain level of temperature is achieved. The resulting device can be used as a temperature trigger sensor that does not require quiescent power to operate. The thermal bimorph could be based on a standard vertical thermal bimorph that moves out-of-plane under temperature loading. However, the temperature trigger sensor for such a device may require complex processing to make contacts and other structures out-of-plane of the microchip. Achieving useful, functional, and complex contact, latching, actuating, and other structures is much simpler on a microchip if performed in the plane of the chip. In order to use those functional structures, a thermal bimorph that moves laterally in the plane of the microchip is required. The present invention is that lateral-moving thermal bimorph. The present invention may be used as a temperature sensitive switch, or in other actuator applications in which lateral movement in response to temperature variation is desired.
Prior inventions have disclosed micro-machined bimorph devices, but none have had the advantages of the present invention in providing lateral movement of a thermal bimorph in response to ambient temperature changes.
For example, US 2004/0084997 A1 discloses a piezoelectric bimorph actuator comprised of two electrorestrictive materials that change length in response to an applied electrical field. This invention claims to provide lateral motion, but the motion is in fact perpendicular to the plane of the materials comprising the bimorph (i.e. vertical rather than in the plane of the surfaces). Also, the components of the bimorph are separately fabricated and then assembled and bonded together, rather than being micromachined in a one-piece structure as is the present invention. This prior art bimorph also requires electrical power to operate.
U.S. Pat. No. 5,382,864 and applications U.S. 2002/0074901 and 2002/0149296 also disclose piezoelectric bimorph actuators that displace vertically in response to an applied electrical field.
Another vertically-moving bimorph is disclosed in US 2004/0164649 A1, which describes a piezoelectric micromachined bimorph in which the two bimorph materials are fabricated separately on separate substrates and then are bonded together.
U.S. Pat. No. 5,463,233 discloses a micromachined thermal switch that uses a thermal bimorph as an electrical switch, but the movement of the thermal bimorph in this invention is also in a vertical direction rather than in the plane of the substrate. Similarly, U.S. Pat. No. 5,917,226 discloses micromachined thermal sensor comprising a thermal bimorph that displaces in a vertical direction.
Finally, U.S. Pat. No. 6,044,646 discloses a micromachined thermal actuator that can move in a direction either in the plane of or normal to the substrate. This actuator, however, does not use a simple bimorph device but employs independently-controllable heaters that require the application of external power to operate.
In sum, none of the prior art patents discloses a micromachined thermal bimorph that displaces in a lateral direction in response to ambient temperature changes.